The present invention relates to clock generation circuits and methods of operation thereof, and more particularly, to phase locked loop (PLL) circuits and methods of operation thereof.
PLL circuits are commonly used in clock generator circuits, such as clock buffers, that are used to generate precisely timed clock signals for distribution to various components of an electronic system. A PLL circuit typically comprises a closed loop that is used to synchronize an output clock signal with an input reference clock signal. It is generally desirable that a PLL be capable of generating an output signal that remains closely synchronized with its reference or master clock (i.e., with minimal jitter and high stability) over a range of temperatures and/or power supply voltages. It may also be desirable for the PLL to be capable of operation over a wide range of input reference clock signal frequencies.
A common issue in designing PLLs is achieving a PLL with both wide frequency range and desirable jitter suppression. For example, some conventional single-loop PLL designs that use voltage controlled oscillators (VCOs) may suffer from a disadvantageous tradeoff of loop bandwidth versus operating frequency range. In particular, as gain of the VCO is increased to increase the frequency range of the PLL, there may be a concomitant increase in susceptibility to jitter.
U.S. Pat. No. 5,942,949 to Wilson et al. describes a PLL circuit having a VCO with multiple selectable operating curves (input voltage versus output frequency transfer functions). During an auto-trim operation, a state machine applies a sequence of digital control input values to the VCO to select a succession of different operating curves until an appropriate operating curve for the present PLL application is identified. Upon identification of the appropriate operating curve, the PLL switches to normal operation using the identified operating curve. Such an approach may be disadvantageous in some applications in which it is desirable to provide dynamic operation of over a wide range of frequencies.